The subject invention relates to film-forming resins capable of low-temperature cure. More particularly, the invention relates to film-forming resins capable of curing through alkoxy silane groups and coating compositions containing the film-forming resin and, optionally, polyols.
In recent years, the coatings industry has become increasingly interested in low-temperature cure coating compositions. These compositions must be capable of being applied by conventional techniques such as spraying, dipping, roll-coating, brushing, etc., yet be capable of curing at a low temperature, ideally room temperature. Obviously, the primary advantage of these compositions is the lessened energy demands.
The coatings industry has also become interested in low organic solvent-containing coating compositions. Such compositions must also be capable of application by conventional techniques. These coating compositions present several advantages including high film build per application, economy of materials, especially organic solvents which are lost to ambient surroundings, lessened concerns about pollution problems caused by the evaporation of organic solvents and also energy savings since less air needs to be heated to evaporate the solvents in a curing oven. Additionally, the organic solvents themselves have been in short supply.
Much work has been expended in the coatings industry to formulate coating compositions which contain a low organic solvent content. One area in particular which has received attention has been in the formulation of water-based coating compositions. Water is itself, of course, quite inexpensive and does not pose environmental problems. However, film-forming resins which are capable of being reduced with water are themselves normally water-sensitive and do not provide a film having all the desired characteristics, such as humidity resistance and durability. Still another area of coating technology which has received attention in recent years is the formulation of high solids coating compositions which are of a low viscosity and can be applied by conventional techniques. Such coating compositions can contain what is referred to as reactive diluents. These reactants are capable of thinning the coating composition and then reacting with the film-forming resins after application to a substrate. In effect, the diluent becomes a part of the cured film.
Additionally, there have been formulated coating compositions which are capable of curing at a low temperature and which are low in organic solvent content. However, the known compositions all have one or more drawbacks associated with them. Thus, for example, coating compositions based on alkyd resins are slow to cure to a durable film and the cured film often does not have particularly good properties. One pack and two pack isocyanate-containing coating compositions suffer from health concerns associated with the isocyanate. Epoxy and amine-containing coating compositions have the disadvantage of having to be a two pack system and the resultant films are less durable than often desired.
One class of film-forming resins which has been extensively used because of its good film properties has been the high molecular weight thermoplastic urethane resins. Compositions made from these resins provide good films and are capable of curing at ambient conditions. One drawback they have is that compositions containing the resins require a high solvent content to give them a viscosity suitable for coating purposes. Another class of film-forming resins with good film properties are those thermoset urethane polyol resins which are mixed with a melamine cross-linking agent. However, a drawback of this system is that a high cure temperature is needed. There have now been found urethane resins which are capable of forming a film at low temperature and, when formulated in a coating composition, high levels of organic solvents are not needed to achieve application viscosity. Compositions containing the film-forming resins of this invention can be applied to a substrate to form a durable film. Components such as polyols and cross-linking agents can be blended with the new film-forming resins to provide additional coating characteristics.
As used herein, all percents and ratios are by weight unless otherwise indicated.